It is known that tertiary amines are catalysts for epoxy systems and can be utilized to accelerate the hardening of liquid or solid epoxy resins. Catalysts and accelerators of this class which are well known for such purposes include benzyldimethylamine, 2-(dimethylaminomethyl)-phenol, 2,4,6-tris(dimethylaminomethyl)-phenol and triethanolamine. These tertiary amines have the common property of being extremely reactive at ambient temperature and thus react strongly in comparatively brief periods of time with the epoxy resin. As a consequence, the pot life of such systems ranges only from several minutes to several hours. Such short pot lives may be inconvenient for many applications. In addition, the tertiary amines mentioned are irritants to the personnel utilizing same and are hygroscopic.
It has been proposed to increase the pot life by the use of salts of tertiary amines in epoxy systems. Tertiary amine salts which have been utilized for this purpose include the tri-2-ethylhexoate of 2,4,6-tris(dimethylaminomethyl)phenol. Such salts by comparison with the tertiary amines from which they derive, have the advantage that they are significantly less reactive without introducing serious drawbacks into the epoxy system and hence can increase the pot life without any accompanying drawbacks of a type which may be detrimental to the product.
However, they are generally of a liquid form which interferes practically entirely with any effort to utilize them in solid epoxy systems. Most of the tertiary amine salts which have been found to be suitable, moreover, are soluble in water, a characteristic which complicates the preparation of the salts. Furthermore, the salts generally possess odors as strong as or stronger than the odors of the amines from which they derive and may be as irritating or more irritating. Perhaps, however, the greatest drawback of such salts is that their hygroscopicity precludes use in any number of applications.
Efforts have been made to develop catalysts or accelerators which are insoluble in water and without the odor characteristic of the compounds previously described. The compounds which have thus been developed, e.g. as described in U.S. Pat. No. 3,497,524, are the tertiary amine salts of tetrachlorophthalic acid and tetrabromophthalic acid. These tertiary amine salts, when utilized as catalysts, permit the preparation of epoxy systems having extremely long pot lives and enable the storage of the composition at ambient temperature for prolonged periods while nevertheless insuring rapid hardening at temperatures of the order of 120.degree. to 250.degree. C.
It appears that with these compounds part of the acid of the composition blocks or deactivates the tertiary amine moiety to promote the cold stability of the system. With heating, however, the salt appears to dissociate with liberation of free tertiary amine to catalyze the epoxy reaction while the acid at the elevated temperature combines with the hydroxyl groups present in the system. As a consequence, there is a formation of ester groups, with each esterification liberating a molecule of water. The formation of water during the hardening phase, while not considerable, is nevertheless a serious drawback for the epoxy system because the trapped water materially reduces the physical and electrical properties of the hardened resin. In part the water which is liberated represents a volatile component. This defeats another advantage of an epoxy system which is the fact that normally such a system is utilized because it is capable of hardening without the volatile material.
French Pat. No. 1,282,310 describes the use of certain substituted ammonium phenates as accelerators of epoxide resin based and polycarboxylate acid anhydride based hardenable mixtures. While such systems may be desirable in many cases where liquid formulations can be used, they have not been found to be effective when the system is in the form of a powder, namely a molding powder or a powder which is to be applied to a substrate from a fluidized bed. These systems, moreover, have significant hygroscopicity, low fluidity in a molten state and low resilience, i.e. high rigidity, which translates into a greater brittleness or fragility than may be desirable.
The use of phenolic novolac resins as hardeners for epoxy systems accelerated by tertiary amines has reduced the problem of hygroscopicity which was formerly associated with the tertiary amines and also eliminates in part the problem of fluidity of the powder. These improved systems, however, have little long term storage capabilities with evolution on storage which results in a rapid loss of fluidity on molding. If one attempts to reduce the amount of acceleration which is present to increase fluidity and storage life, one tends to reduce the speed of hardening to a level which is incompatible with industrial molding operations.